The present invention generally relates to secondary power sources such as fuel cells and more particularly to back-up power systems that utilize batteries or fuel cells to provide uninterrupted power to a critical load upon failure or interruption of the primary power source.
Power plants for supplying direct current (DC) electrical power are common throughout the world, and are often used to power critical computing, communications, and control equipment, as well as for many other uses. A typical DC power plant includes one or more rectifiers for converting alternating current (AC) from the electrical grid into DC power, which is fed to an electrical bus. The equipment using the power, the load, is connected to this bus and draws power from it. Typically, batteries are also connected to the bus in order to provide backup power should the primary AC power source or the rectifiers fail. Various types of batteries can be used for this purpose, such as lead-acid, nickel-cadmium, lithium-ion, and others.
Additionally, it is possible to directly supply power to the DC bus from another secondary power source, or use as a back-up power source on the DC bus, a fuel cell, a solar panel, a windmill, a DC generator using an internal combustion engine, or other power sources. An advantage of such secondary power sources over batteries is that they often are capable of providing far longer backup times. However, unlike the batteries, these secondary power sources sometimes cannot provide power on short enough notice (or instantaneously) to provide uninterrupted power to the load upon a failure of the primary AC power. Back-up power plant systems that have the ability to control and take advantage of different power sources may be attractive. Back-up power systems may also be set-up to handle short primary outages differently than ultra-long power outages. Such control systems could reduce power requirements of the load in a given outage, thereby reducing the power requirements of the secondary power sources, and potentially lowering the cost and size of such power sources. A system or method of operating a DC power plant designed to take advantage of these possibilities would be economically attractive.
A secondary power source may be used to supply power to the DC bus with or without batteries. In either case, secondary power sources such as fuel cells, windmills, propane generators, and others typically require electronic conditioning of their output power before it can be fed to the DC bus. A system or method designed to provide conditioned power that is compatible with the present conventions and DC power plant design would be useful.
Fuel cell technologies have been rapidly improving and are becoming attractive secondary power source candidates for integration into applications. There are a variety of fuel cell technologies that can be considered as secondary power sources. Depending on design and technology, cells could use hydrogen, zinc, aluminum, methanol, and other types of hydrocarbons as fuel. A regenerative fuel cell is a fuel cell in which the fuel can be electrically recovered from the reaction products of the fuel cell discharge reaction. The fuel can then be re-used. In the case of a hydrogen regenerative fuel cell, the reaction product of the fuel cell discharge is water (H2O), which can be electrolyzed, or regenerated, back into fresh hydrogen fuel using electricity. In the case of a zinc regenerative fuel cell, the reaction product of the fuel cell discharge is zinc oxide (ZnO) or other zinc-containing reaction products, which can electrolyzed, or regenerated, back into fresh zinc fuel using electricity. For a regenerative fuel cell, the regeneration apparatus can be separated from the fuel cell or packaged together with the fuel cell. Integrated regenerative fuel cells are a desirable configuration for back-up power applications since existing back-up systems deploy rechargeable batteries.
Metal-air fuel cells are described further in U.S. Pat. No. 6,296,958 to Pinto et al., entitled xe2x80x9cRefuelable Electrochemical Power Source Capable Of Being Maintained In A Substantially Constant Fuel Condition And Method Of Using The Same,xe2x80x9d and U.S. Pat. No. 5,952,117, entitled xe2x80x9cMethod And Apparatus For Refueling An Electrochemical Power Source,xe2x80x9d both of which are incorporated herein by reference. For additional information on this embodiment of a zinc/air battery or fuel cell, the reader is referred to U.S. Pat. Nos. 6,153,328; and 6,162,555, which are hereby incorporated by reference herein as though set forth in full.
Although fuel cells can provide continuous power to a load for longer periods of time, as opposed to a conventional battery back-up source, there are issues with fuel cells that make them more difficult to integrate into traditional back-up systems when compared to lead-acid batteries. For example, fuel cells need controller and control systems to perform operations such as the movement of fuel, whereas batteries can be deployed without controllers. Furthermore, it is generally difficult for fuel cells to provide instant startup at full or rated power when compared to a conventional lead-acid battery. Even if a fuel cell could provide instant start-up, it would have to have an output impedance similar to a battery in order to be able to handle sudden high-power requirements such as clearing fuses and circuit breaker faults. Although fuel cell technology may advance to address these issues, today they need to be addressed in order to integrate fuel cells as power sources in back-up applications. Therefore, there is a need for a system and an apparatus that can integrate a power source such as a fuel cell into conventional back-up systems, in order to make use of its advantages such as the capability of providing long-term back-up power.
According to one aspect of the invention, a regulated DC power supply includes a rectifier coupled between a primary AC power source and a DC bus, the rectifier adapted to convert AC power from the AC power source to DC power for a load that draws power from the DC bus. The power supply also includes a fuel cell arrangement electrically coupled to a power converter that is coupled to the DC bus, wherein the converter is adapted to condition an unconditioned DC power output of the fuel cell. The conditioned DC output of the converter is coupled to the DC bus that powers the load. In addition, a fuel cell controller is communicatively coupled with the fuel cell arrangement and the rectifiers arrangement and is adapted to initiate DC power output from the fuel cell arrangement upon detecting an AC power outage. The fuel cell controller is further adapted to disengage the fuel cell arrangement upon detecting AC power resumption. In another embodiment of the invention, the rectifier arrangement may also incorporate a system controller central to the rectifiers, and the fuel-cell controller may also communicate with this system controller.
In a related embodiment, the fuel cell in the DC power supply is regenerative and can be of the type that is metal/air or hydrogen. A regenerative fuel cell (RFC) comprises a fuel storage component, a reaction-product storage component, a fuel cell stack that produces electrical power from the electrochemical reaction of the fuel (typically zinc or hydrogen), an oxidant (typically oxygen from the air), and a regenerator or electrolyzer that uses electrical power from a primary source (such as the electrical grid) to convert the reaction product (such as water, zincate, or zinc oxide) back into fuel (such as hydrogen or zinc) and the oxidant (such as oxygen).
In another embodiment, the DC power supply includes a plurality of rectifiers and power converters and optimally includes a battery arrangement electrically coupled to the DC bus to provide uninterrupted DC power to the load upon an AC power source outage. The battery arrangement enables the voltage on the DC bus to be sustained throughout the initial phase of the AC power outage during which the fuel cell system is preparing itself to supply full power to the DC bus. This system would solve the instant start-up problem generally associated with fuel cells provided that the start-up time of the fuel cell is relatively short, i.e., a few minutes or seconds or less as opposed to several hours.
In some applications it is desirable to avoid activating the fuel cell for short outages (for example, a 5-second outage), and instead the backup power needs can be handled by sizing battery arrangement such that it handles the initial start-up and appropriate amount of short outages. Further, the battery arrangement integration solves the potential mismatch in impedance as the battery provides additional current to clear fuses and breaker faults in the distribution network on the DC bus or in the load. The battery arrangement can also supply sufficient current to power the full load for a pre-determined period while the load is partially curtailed before switching over to fuel cell power. This is particularly important where the fuel cell is only designed to power part of the load during an AC power outage to reduce the power requirements (and hence the cost and size) of the fuel cell. Such a system could make it more economically feasible to keep a part of a load, such as the more critical communication and control functions that the DC plant may be powering, available for longer back-up times, while utilizing the battery arrangement on the full plant load for the shorter back-up times.
According to another aspect of the invention, a method of providing extended backup power to a load via a power plant having a battery backup arrangement includes providing DC power to the load from the battery arrangement via a DC bus upon detecting an interruption in a primary power source. A portion of the load is shed or reduced when the battery voltage reaches a predetermined transition voltage and then power is provided on the DC bus at a predetermined voltage from a fuel cell arrangement. A fuel cell controller is used to share current between the battery arrangement and the fuel cell arrangement, wherein the fuel cell controller reduces the current load on the battery. Upon detecting primary power source resumption, the fuel cell controller switches the load from the fuel cell and battery arrangement to the primary power source.